1. Field of the Invention
This invention relates to heat exchangers and more particularly, to plates and methods for producing the plates that support the heat exchanger tubes in the riser chamber between opposing tube sheet faces, and the like.
2. DESCRIPTION OF THE PRIOR ART
Apparatus of very large size is an almost characteristic feature of present day industrial activity. Heat exchangers in many chemical processing plants, power generation facilities, and marine propulsion units, for instance, typify this trend toward large sizes that almost necessarily follows from the requirement to handle great quantities of fluids.
For example, in a pressurized water nuclear power station the steam or vapor generators that transfer the reactor-produced heat from the primary coolant to the secondary coolant that actually drives the plant turbines, may be as long as 75 feet and have an outside diameter of about 12 feet. Within one of these heat exchangers, the tubes through which the primary coolant flows each might be no more than 5/8 inch in outside diameter, but have an effective length of 52 feet between the tube-end mountings in the opposing faces of the tube sheets. Typically, there may be a bundle of more than 15,000 tubes in one of these heat exchangers. It is clear that there is a need to provide structural support for these tubes in the span between the tube sheet faces to insure tube separation, adequate rigidity, and the like.
In the past, lattices positioned at generally regular intervals in the tube bundle between the tube sheets have been used to provide this support. These lattices usually comprise open cellular frameworks that are built-up from interlocking metal strips. The strips that form the cells engage the sides of the tubes which are lodged within the respective lattices. These lattice structures are not entirely satisfactory because they lack inherent strength and lead to a number of manufacturing difficulties.
Another approach to solve the tube support problem has led to the development of a drilled plate structure. This support system consists of an array of flat plates that is arranged in the heat exchanger with the planes of the individual plates transverse to the longitudinal axes of the tubes in the bundle. Holes are drilled in each of the plates to accommodate the tubes. Secondary coolant fluid communication is established through these plates by drilling additional holes in those plate portions that are in the midst of each cluster of three tubes. Although these drilled plates are stronger than the lattice structure, the tube receiving holes each must have a slightly larger diameter than the outside diameter of the associated tubes in the bundle. This size difference is provided because the individual tubes usually are not perfectly straight and the plate holes are not in precise alignment. Thus, some tolerance is necessary to accommodate these manufacturing variations during heat exchanger assembly. Because of these small differences, the centerlines of the individual tubes are not in alignment with the centers of the respective holes. This causes the individual tubes to rest against one side of a hole and leave a thin crescent shaped gap between the tube and the balance of the surface of the hole in question.
The points at which the crescent edges terminate, however, tend to promote an undesirable condition that frequently is referred to as "crevice corrosion." The term crevice corrosion is generic to a number of physical and mechanical effects, many of which are not well understood. In any event, crevice corrosion leads to a deterioration of the tube and plate structure.
Illustratively, one type of crevice corrosion in the gaps that are adjacent to the crescent ends is believed caused by the evaporation of successive droplets of secondary coolant. These droplets form on the metal surfaces in the narrow end spaces. The heat transfer process causes the droplets to evaporate and leave behind, in many cases, a small residue of solid matter. The process is repeated a number of times and produces an accumulation of solid material that ultimately pits or corrodes the metal surfaces. Damage of this sort causes expensive and time-consuming repairs, or, if unchecked, it can cause a tube failure.
Another type of heat exchanger tube support plate has been suggested and is referred to as the "cloverleaf" design. The cloverleaf plates are perforated, each perforation having inwardly directed spaced fingers that touch the surface of the associated tubes. Although the direct contact between tube and plate eliminates crevices, and thence the source of crevice corrosion that characterizes the drilled plate structure, the design nevertheless is unsuitable for application to large heat exchangers of the general dimensions described above because there is no clearance to allow for the cumulative effect of non-linear tubing and slight misalignments in the perforations.
Accordingly, there is a need for a sturdy heat exchanger tube support plate that does not promote corrosive attack. The plate, moreover, should not create an obstacle to the efficient assembly of the heat exchanger.